Microstructural evolution during friction stir welding of Ti-6Al-4V alloy

S. Mironov, Yan Zhang, Yutaka Sato, Hiroyuki Kokawa

Research output: Chapter in Book/Report/Conference proceedingChapter

1 Citation (Scopus)

Abstract

The successful application of the Ti-6Al-4V alloy relies heavily on the availability of welding process producing superior quality joints. Fusion welding technologies, which are conventionally used to join titanium alloys, generally enable to produce high-quality welds with a relatively high productivity. However, the fusion welding is usually associated with solidification problems which may significantly deteriorate service properties of the titanium weldments. In this regard, friction stir welding, being an innovative "solid-state" joining technology, may potentially be attractive for Ti-6Al-4V. Basically, FSW involves plunging a specially designed tool rotating at high speeds into abutted edges of sheet workpieces to be welded and then translating the tool along the weld seam. The rotating tool produces frictional heat which softens the material so it can be readily extruded around the tool to fill the cavity to the rear of the tool and to form a joint in a solid state. As a solid-state process, FSW avoids (or limits) solidification problems and thus provides defect-free welds having excellent mechanical properties. In order to improve the basic physical understanding of FSW of Ti-6Al-4V, the present contribution is focused on microstructural evolution during the welding process. The microstructural development is found to be a complex process involving α-to-β phase transformation during heating stage, deformation in the high-temperature β-phase field during stirring stage and β-to-α phase transformation upon cooling stage of FSW. The α-to-β phase transformation is shown to be governed by epitaxial growth of the pre-existing β phase consuming the α phase. Reconstruction of the β-grain structure and orientation measurements in the retained β phase have demonstrated that microstructural evolution in the high-temperature β phase field is a complex process involving grain growth, geometrical effects of strain, transverse grain subdivision and discontinuous recrystallization stimulated by local grain boundary migration; material flow may be described in the term of simple shear deformation arising from {110}<111> slip. The β-to-α phase transformation after FSW is established to be governed by Burgers orientation relationship which dictated the final weld crystallography.

Original languageEnglish
Title of host publicationTitanium Alloys
Subtitle of host publicationPreparation, Properties and Applications
PublisherNova Science Publishers, Inc.
Pages385-403
Number of pages19
ISBN (Print)9781608761517
Publication statusPublished - 2010 Dec 1

Keywords

  • Friction stir welding
  • Phase transformation
  • Ti-6Al-4V alloy

ASJC Scopus subject areas

  • Materials Science(all)

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